The standard notation for a right triangle!

For a while we'll be looking mainly at right triangles, so it would be useful to use a standard notation for the angles and sides of these triangles.

Consider a right triangle ABC with a right angle at C. We'll generally use the letter a to denote the side opposite angle A, the letter b to denote the side opposite angle B, and the letter c to denote the side opposite angle C, that is, the hypotenuse.
With this notation, sin A = a/c, and sin B = b/c.
Next we'll look at cosines. Cosines are just sines of the complementary angle. Thus, the name "cosine" ("co" being the first two letters of "complement"). For triangle ABC above, cos A is just sin B.